Efficient Forward‐Bias Bipolar Membrane CO2 Electrolysis in Absence of Metal Cations

Alkali-metal cations are assumed to be indispensable for CO₂ reduction, but often lead to undesired failure mechanism such as salt precipitation due to the non-electrochemical CO₂ conversion to (bi)carbonates. A promising alternative CO₂ reduction cell design involves the use of pure water and a metal cation-free forward-bias bipolar membrane configuration interfaced to a non-metallic single-atom NiNC catalyst. We report such a CO₂ electrolysis cell design with unprecedented product selectivities in absence of metal cations.

Abstract

The acid-base reaction of CO₂ with hydroxide ions to (bi)carbonate anions at the cathode of alkaline exchange membrane (AEM) CO₂ electrolyzer has detrimental impact on their performance. (Bi)carbonate buffers the local cathode pH, and in combination with metal cations, may lead to precipitation of salts at the cathode. This non-electrochemical conversion of CO₂ significantly reduces the CO₂ utilization efficiency and limits the CO₂ single pass conversion of AEM CO₂ electrolyzer to 50% if CO is desired. Acidic metal cation-free CO₂ electrolysis has the potential to address and mitigate these problems. Here, CO₂ valorization is demonstrated at faradaic CO efficiencies (FE) of up to 80% FE_CO in forward-bias BPM cell architectures using actual neutral pure water feeding at the anode. This study demonstrates how immobilized anion exchange ionomer layers thereby facilitate the metal cation-free CO₂ valorization thanks to their positively charged functional NR₄ groups. Unlike metal cations, the immobilized positively charged groups are not washed out of the reactor. This study shows that careful design of the distribution and location of the anion exchange ionomer molecules within the Gas Diffusion Electrode is key to efficient CO₂-to-CO electrolyzer cell.